1. University of Surrey, Guildford
Film Formation of Waterborne Pressure-Sensitive Adhesives
Joseph Keddie
Department of Physics,
University of Surrey, Guildford
3 November, 2004

2. Pressure Sensitive Adhesives (PSAs)
• PSAs are aggressively and permanently tacky at
room temperature, adhering under light pressure.
• Usually a polymer melt at room temperature (Tg< -30 °C)
• Used in graphic arts
• Used in medical applications
• Used in tapes and labels

3. Why are PSAs so sticky?
• Soft polymers can achieve intimate contact with a rough substrate, leading to mechanical interlocking.
• With close contact (D ~ 0.2 nm) between surfaces, the van der Waals pressure become significant: P ~ A/(6pD3) ~ 7000 atm!
• Usual polar or acid/base interactions between the PSA and the substrate, depending on chemistry. But there is no covalent bonding.

4. Energy Dissipation in PSA De-bonding
vd = µm/s F F V
= 30 µm/s d
Contact t c
= 1 s P c
= 1 MPa d
Lakrout, H.; Sergot, P.; Creton, C.
J. Adhes. 1999, 69,
Lakrout, H.; Creton, C.; Ahn, D.; Shull,
K. R. Macromolecules 2001, 34,

5. Environmentally-benign
Key Challenges in PSAs
• Trend towards waterborne,
colloidal PSAs
Reduced VOCs
Environmentally-benign
• Trend towards clear labels

6. Environmentally-benign
Key Challenges in PSAs
• But the adhesion strength and
water resistance of waterborne
PSAs are poor!
Reduced VOCs
Environmentally-benign

7. Key Challenges in PSAs After soaking in water for 10 min.:
• But the adhesion strength and
water resistance of waterborne
PSAs are poor!
After soaking in water for 10 min.:
Poor water resistance
Good water resistance

8. Why? Key Challenges in PSAs
• There is a clear need to characterise PSA
morphology and relate it to film formation
mechanisms: Aim of our work
Poor water resistance
Good water resistance

9. Dodecahedral structure
Idealised View of Latex Film Formation
Polymer-in-water dispersion
Close-packing of particles
Water loss
Dodecahedral structure
(honey-comb)
Deformation
of particles
Interdiffusion
and coalescence
Homogenous Film

10. Typical Morphologies
Particles are deformed to fill all available space: rhombic dodecahedra
Y. Wang et al., Langmuir 8 (1992) 1435.

11. Example of Good Coalescence
Environmental SEM
Immediate film formation upon drying!
Tg of latex  5 °C; film-formed at RT
1 mm
Hydrated film
J.L. Keddie et al., Macromolecules (1995) 28,

12. Experimental Evidence for Vertical Non-Uniformity during Drying
Densely-packed particle layer
Cryogenic SEM
E. Sutanto et al., in Film Formation in Coatings, ACS Symposium Series 790 (2001) Ch. 10

13. Theory: Peclet number for vertical drying
Competition between Brownian diffusion that re-distributes particles and evaporation that causes particles to accumulate at the surface

14. Peclet number for vertical drying uniformityH R
Dilute limit
Pe << 1

15. Simulations of the Vertical Distribution of Particles
pol
Vertical Position
Pe = 0.2
Close-packed
Simulations by A.F. Routh
Top

16. Simulations of the Vertical Distribution of Particles
pol
Vertical Position
Pe = 1
Close-packed

17. Simulations of the Vertical Distribution of Particles
Vertical Position
pol
Pe = 10
A.F. Routh and W.B. Zimmerman, Chem. Eng. Sci., 59 (2004)

18. Driving Force for Particle Deformation
Energy “gained” by the reduction in surface area with particle deformation is “spent” on the deformation of particles:
Deformation is either elastic, viscous (i.e. flow) or viscoelastic (i.e. both).
For coalescence of 1 L of latex with a 200 nm particle diameter (50% solids), there are ~1017 particles and DA = -1.3 x 104 m2. With g = 3 x 10-2 J m-2, then DG = J.

19. Particle Deformation Mechanisms
Dry Sintering: gpa
Wet Sintering: gpw
Capillary Action: gwa
Skin Formation r

20. Latex Film Formation Mechanisms and Vertical Homogeneity
Dry Sintering: pa
10000
Receding Water Front
100
Capillary Deformation: wa
Partial Skinning
PSAs! 1
Wet Sintering: pw
Skinning 1
See A.F. Routh & W.B. Russel, Langmuir (1999) 15,

21. Atomic Force Microscopy (AFM) of PSAs
• Very difficult because
(1) Polymer melt surface is very easily indented
(2) By definition, the surface is very sticky!
Atomic Force Microscopy (AFM) of PSAs
Ao : “free” amplitude
Asp : “setpoint” amplitude
dsp : tip-surface distance
zind : indentation depth
Asp=dsp+zind Ao
(>Asp)
dsp/Ao = rsp < 1
rsp : setpoint ratio
• Requires careful control and optimisation of
tapping parameters:

22. Discrete Particles Observed at PSA Surface!
Silicon tip, k = 48 N/m, fo = 360 kHz
PSA latex
Tg = -33ºC
(bimodal particle size)
looptack on glass
=512 N/m
Ao=163nm
dsp=75nm
rsp=0.46
acrylic
latex
Tg = 20ºC
non-sticky
surface
Ao=18nm
dsp=15nm
rsp=0.83
Top views
3mm x 3mm
scans
Vertical scale = 200nm
Vertical scale = 50nm
Slice views
1mm x 1mm
scans

23. Apparent Surface Topography is Sensitive to
Free Amplitude and Setpoint Ratio
Same Surface
Ao=163nm dsp=75nm
rsp= Ra=6.9nm
Ao=123nm dsp=61nm
rsp= Ra=5.8nm
Ao=98nm dsp=50nm
rsp= Ra=4.7nm
Ao=72nm
dsp=53nm
rsp=0.73
Ra=2.6nm
Ao=38nm
dsp=35nm
rsp=0.92
Ra=1.2nm

24. Amplitude-distance curve obtained from
a PSA surface prone to indentation,
showing a calculation of the indentation depth.
Bar et al., Surface Science,
457 : L404-L412 (2000).

25. Amplitude-distance curves are used to characterise
the tip-sample interactions
Lessons:
• The surface is indented very deeply!
• Tip adheres to surface at tapping amplitudes < 35 nm.
Hard surface

26. Minimal indentation with a low amplitude and high setpoint ratio
Ao=163nm dsp=75nm
zind=74nm
Ao=123nm dsp=61nm
zind=44nm
Ao=98nm dsp=50nm
zind=30nm
Ao=72nm
dsp=53nm
zind=19nm
Ao=38nm
dsp=35nm
zind=3nm
Minimal indentation with a low amplitude
and high setpoint ratio
If Ao < 35 nm, energy of tapping is low and tip sticks to surface!

27. Ao=135nm dsp=115nm rsp=0.85 zind=18nm
Indentation
leads to artefacts !
(1mm x 1mm scans)
height scale = 50nm
Ao=135nm dsp=86nm
rsp=0.63 zind=44nm
• When the indentation depth is small, surface topography is less likely to be altered.
• Using optimised tapping conditions, cylindrical particles
are observed, surrounded by a liquid-like medium.
See Mallégol et al., Langmuir (2001) 17, 7022.

28. The second phase is water-soluble
Acrylic (EHA-BMA-MMA…) PSA film formed at 60ºC (3 min)
Same PSA film after rinsing with water for 1 min.
Water
Phase contrast image
1mm
Water-soluble phase is likely to be surfactant and free polymer fragments.

29. RBS Evidence for Surfactant Excess at the Adhesive/Air Interface
0.08 at% Na
0.09 at% S
0.03 at% K
60 nm layer
< particle diam.
Used a scanning mbeam with low current (5 nA) on a cryogenic stage C O
See Mallégol et al., Langmuir (2002) 18, p 4478.

30. Stabilisation of the Latex Particles against Coalescence
Water
Structure might be analogous to that of a biliquid foam, as has been observed in concentrated emulsions.
See Crowley T.L. et al. Langmuir (1992) 8, 2110 and Sonneville-Aubrun et al. Langmuir, (2000) 15, 1566

31. Effect of “Cleaning” Latex Serum
PSA film formed from a diluted bimodal dispersion
PSA film formed from a bimodal dispersion “cleaned” via dialysis
Image sizes: 5 mm x 5 mm; Height mode on left; phase mode on right

32. Interface with Silicone Substrate
The Morphology of the Air Surface Differs Strongly from that at the Interface with the Substrate
Air Surface
Film formation at 60 °C
Interface with Silicone Substrate
5 mm x 5 mm scan

33. Particles are Stable under the Application of Shear Stress
Image of surface acquired between 4 and 11 min. after shearing
Acquired between 11 and 18 min. after shearing
Scan size: 5 mm x 5 mm
J. Mallégol et al., J. Adh. Sci. Tech. (2003)

34. P. M. Glover, et al., J. Magn. Reson. (1999) 139, 90.
How and why are the solids in the latex serum transported to the film surface?
Need for water concentration profiles during drying….
GARFIELD
P. M. Glover, et al., J. Magn. Reson. (1999) 139, 90.

35. GARField: A Magnet for Planar Samples
A low cost, permanent magnet with shaped pole pieces for the high resolution profiling of films.
P. M. Glover, et al., J. Magn. Reson. (1999) 139, 90.

36. GARField depth profiling magnet
Gradient At Right-angles to the Field
Characteristics :
0.7 T permanent magnet (B0)
17.5 T.m-1 gradient in the vertical direction (Gy)
Abilities :
accommodates samples of 2 cm by 2 cm area
achieves better than 10 m pixel resolution! B0 Gy B1
Film Sample
Coverslip
RF Coil
position
Gravity
Signal intensity

37. J.-P. Gorce et al., Eur Phys J E, 8 (2002) 421-29.
Dependence of Water Concentration Profile on Pe
High humidity Pe  0.2
H = 255 m, E = 0.2 x 10-8 ms-1, D = 3.23 x m2s-1
Uniform water concentration profiles
J.-P. Gorce et al., Eur Phys J E, 8 (2002)

38. J.-P. Gorce et al., Eur Phys J E, 8 (2002) 421-29.
Dependence of Water Concentration Profile on Pe
Flowing Air Pe  16
H = 340 m, E = 15 x 10-8 ms-1, D = 3.23 x m2s-1
Non-uniform water concentration profiles
J.-P. Gorce et al., Eur Phys J E, 8 (2002)

39. J.-P. Gorce et al., Eur Phys J E, 8 (2002) 421-29.
Dependence of Water Concentration Profile on Pe
Still air and higher viscosity Pe  16
H = 420 m, E = 8 x 10-8 ms-1, D = 1.94 x m2s-1
Non-uniform water concentration profiles
J.-P. Gorce et al., Eur Phys J E, 8 (2002)

40. Simulated Water Profiles with Various Types of Film Formation
Dry Sintering:
Water recedes from the film surface
Capillary deformation:
Water is always near the film surface

41. Drying Profiles in Other Waterborne Films
Acrylic Latex near Tg:
Uniform water recession from surface
Time
Height (mm)
Low-Tg Alkyd Emulsion:
“Skin” formation
Height (mm)

42. MR Profiles of PSA Drying
• Linear water concentration gradients
• Surface always wet
Height (mm)
Drying delayed by 14 min.
Drying delayed by 82 min.
• Pathway for surfactant and latex serum to be drawn to the film surface
Height (mm)

43. Influence of Drying Rate on Morphology of Air Interfaces
Very slow drying at 8 °C in high humidity: low Pe
Fast drying at 100 °C in a thicker film (400 mm): high Pe
5 mm x 5 mm scan

44. Influence of Drying Conditions on the Surface Excess of Surfactant
Slower drying  More uniform water distributions  Greater surface excess

45. Tackifiers in PSAs • “Tackifiers” are added to PSAs to increase tack.
• Tackifiers are typically a rosin ester or rosin-derivative with a relatively high Tg ( 20 °C).
• They function as “solid solvents” in acrylics.
• Their effect is to reduce the storage modulus (G’) at high temperature but to increase it at lower temperatures. Tackifiers also increase the Tg of PSAs.
• Polymer flow is enhanced and resistance to bond rupture is increased.

46. Tacolyn® 3189 - Eastman Chemical
Effects of Tackifier on Film Morphology
Concentrations of Tackifier:
a = 0%
b = 5%
c = 10%
d = 25%
e = 50%
Tacolyn® Eastman Chemical
Particle identity is progressively lost!

47. Effect of Tackifier on Water Loss Rate in PSA films
The addition of tackifier strongly slows down drying.

48. Tackifier concentrations:
MR Profiles of PSA/Tackifier Drying
Evidence for “skin formation” with increasing amounts of tackifier
Tackifier concentrations:
a = 0%
b = 10%
c = 25%
d = 50%
e = 75%
f = 100%

49. Conclusions
• Particle coalescence does not occur near the surface of low-Tg waterborne acrylic PSAs.
• Surfactant excess near the surface, identified with Rutherford backscattering spectrometry (RBS), stabilises the particles against coalescence.
• Drying profiles, determined with MR profiling, are consistent with particle deformation under the action of capillary pressure.
• Tackifier alters the drying mechanism and promotes “skin” formation in PSAs.
• MR profiling is an ideal complementary technique to AFM and RBS.

50. Collaborators • Dr Jacky Mallégol: all PSA experiments
• Dr Jean-Philippe Gorce: MR profiling of alkyd emulsions
• Dr Olivier Dupont (UCB Chemicals, Drogenbos): latex synthesis and complementary characterisation
• Professor Peter McDonald (University of Surrey): support and advice on MR profiling
• Dr Chris Jeynes (Surrey Ion Beam Centre): RBS

51. Funding • UCB Chemicals, Drogenbos (now “Surface Specialties”)
• “Pump-Priming” Grant for initial access to Surrey’s Ion Beam Facility
• UK Engineering and Physical Science Research Council for recent grant for access to the Surrey Ion Beam Facility
• ICI Paints, Slough

52. Tackified acrylic PSAs
Ex: WB PSA (UCBA Tg~ -40°C (DSC))
with 25wt% (dry/dry) compatible stabilised rosin ester dispersion
(Tacolyn®3189 softening point = 70°C)
DMA in Tensile mode
0.01
0.1 1 10
-60
-40
-20 20 40 60 80
100
T (°C)
tan d
1000
10000
Storage Modulus (MPa)
UCBA -F
UCBA
lower T° » Tg (or low strain rate)  polymer flow, bond formation
higher Tg, T° ~ Tg ( higher strain rate)  resistance to debonding
higher tan T° ~ Tg  energy dissipation upon debonding